The invention relates to that aspect of ophthalmic surgery which is concerned with operations upon the external surface of the cornea.
Operations of the character indicated include corneal transplants and keratotomies; such operations have traditionally required skilled manipulation of a cutting instrument. But, however keen the cutting edge, the mere entry of the edge into the surface of the cornea necessarily means a wedge-like lateral pressure against body cells displaced by the entry, on both sides of the entry. Such lateral pressure is damaging to several layers of cells on both sides of the entry, to the extent impairing the ability of the wound to heal, and resulting in the formation of scar tissue.
My original patent application Ser. No. 552,983, filed Nov. 17, 1983, includes a background discussion of the effects of various available wavelengths of laser radiation in ophthalmic surgery and, in particular, surgery performed on the anterior surface of the cornea. It is explained that radiation at ultraviolet wavelengths is desirable by reason of its high photon energy. This energy is greatly effective on impact with tissue, in that molecules of tissue are decomposed on photon impact, resulting in tissue ablation by photodecomposition. Molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate; the mechanism of the ablation is photochemical, i.e., the direct breakdown of intra-molecular bonds. Photothermal and/or photocoagulation effects are neither characteristic nor observable in ablations at ultraviolet wavelengths, and cell damage adjacent the ablation is insignificant.
Said related-case applications deal with various concepts whereby laser radiation at ultraviolet wavelengths of 200-nm or less are controlled in delivery of laser radiation to the visually used area of the anterior surface of the cornea so as to penetrate the stroma and achieve a predeterminable volumetric removal of corneal tissue, thereby so correctively changing the profile of the anterior surface as to reduce a myopia, or a hyperopia, or an astigmatic abnormality which existed prior to such laser surgery.
Said related-case applications also deal with sculpturing penetration of the stroma, and my copending applications Ser. No. 049,333, filed May 13, 1987 and Ser. No. 059,617, filed Jun. 8, 1987, deal with manipulative and other operations upon the cornea in preparation for and in conjunction with sculpturing operations as in said related-case applications. The present application deals with sculpturing per se, and it will be understood that the manipulative and preparatory and other operations of said Ser. Nos. 049,333 and 059,617 are presently preferred in connection with the sculpturing method and means to be described herein.
The sculpturing technique of parent application Ser. No. 891,285 may be briefly stated as involving corneal exposure to a laser beam of varying spot size to achieve an ablated change in anterior-surface curvature in the visually used central area of the cornea. Various means have been described to achieve this result, and most conveniently the previously described means have involved microprocessor means to assure a given exposure program in the course of which spot-size area varies as predetermined to achieve a given profile change.